Fibrinolysis and plasminogen activation kinetics by tPA (circles), K1K1-tPA (triangles), and delF-tPA (squares) in fibrin clots prepared using different thrombin concentrations. Thrombin concentrations used to form the clots were 5nM (A, fibrin₅) and 100nM (B, fibrin₁₀₀). Representative data using 0.075nM tPA are shown for plasmin generation measured by hydrolysis of S-2251 at 405 nm (positive absorbance change) and simultaneously for lysis of fibrin clots (negative absorbance change, in the absence of S-2251). Large open symbols represent the time points to 50% lysis of each clot; and the solid symbols show the time points for 100% lysis. Small symbols are included on plasminogen activation curves for identification purposes. The 3 plasminogen activation curves for the tPA variants in panel B are very close, indicating very similar activities. The arrows show the times for 50% and 100% lysis for tPA. The inset is a magnified view focusing on the initial rates of fibrinolysis in fibrin₅.